Evidence was obtained that cortical-type (CD4+8+) thymocytes are end stage cells, and not the precursors of other types of T- cell: (1) transfer of large numbers (108) of highly purified CD4+8+ thymocytes I.V. to irradicated congenic recipients; (2) intrathymic transfer of CD4+8+ thymocytes failed to generate any detectable donor-derived cells in the recipients; (3) treatment of mice in vivo with anti-CD8 antibodies had no effect on production of CD4+8- cells; and (4) CD3+, CD4+8+ thymocytes appear late in ontogeny (day 18) after CD4+8-, CD4-8-, and CD4-8+ cells. Finally, we have preliminary evidence that CD4+8- cells may give rise to CD4+8+ cells. We have identified a new type of T cell with a unique T cell receptor. The receptor, designated TRC gamma delta is a heterodimer with disulfide-linked components of 35 and 45 KD, is noncovalently linked to the same CD3 components found in TRC alpha beta-bearing cells, and is precipitated either with monoclonal anti-CD3 or an antiserum to a gamma peptide. These cells can be grown in vitro with a mixture of lymphokines. TCR gamma delta cells are the earliest TCR-bearing cells in fetal ontogeny. We believe that some of these cells are functional and that they represent a distinct lineage from those bearing TCR alpha beta. We have demonstrated the appearance late in thymic ontogeny of a group of cells which is CD4-8- and in which a single TCR V beta family (V beta 8) predominates.